Peel and cut the potatoes into small pieces, dice courgette and carrot. Finely slice leek and celery.

Cook the leek over low heat in a pan with 1 tablespoon of oil for about ten minutes.

Heat 1 tablespoon of olive oil in a large enough pan, and add potatoes, celery, courgette and carrot. Leave to flavor for a few minutes, stirring. Pour a liter of water, and salt and cook over low heat for about 40 minutes, stirring occasionally. After the time, blend half of it with an immersion mixer. Depending on the desired consistency, add water. Add the stewed leek, season with salt and pepper and cook over low heat for 5 minutes after boiling.

Meanwhile, cut the bread into cubes, put it in a non-stick pan, season with oil, salt, pepper and if desired chopped rosemary and cook over low heat for some minutes, stirring.

Fry the bacon without oil in the same pan, turning it on both sides. Cut it into strips and keep it aside. In the same pan, toast some sesame seeds, over low heat, stirring constantly.

Pour your soup into the bowls, sprinkle with a nice handful of thyme leaves and serve with the bacon, sesame and warm croutons.